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The Largest Known Maars on Earth, Seward Peninsula, Northwest Alaska JAMES E

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The Largest Known Maars on Earth, Seward Peninsula, Northwest Alaska JAMES E ARCTIC VOL. 49, NO. 1 (MARCH 1996) P. 62–69 The Largest Known Maars on Earth, Seward Peninsula, Northwest Alaska JAMES E. BEGÉT,1 DAVID M. HOPKINS1 and STEVEN D. CHARRON2 (Received 23 January 1995; accepted in revised form 5 October 1995) ABSTRACT. The Espenberg Maars on the northern Seward Peninsula of Alaska were formed by a series of Pleistocene basaltic eruptions through thick permafrost. The maars were excavated as much as 300 m into older lithologies; ranging from 4 to 8 km in diameter, they are the four largest known maars on earth. Hydromagmatic eruptions which derive water from ground ice are evidently extremely explosive. The high heat capacity of ice in permafrost modulates the supply of water interacting with magma during the eruption, producing consistently low coolant-to-fuel ratios in an environment with a sustained, abundant water supply. The Espenberg Maars demonstrate that, under certain conditions, eruptions which involve the interaction of lava and permafrost are powerful enough to produce craters as large as small calderas. Key words: maar, permafrost, arctic, Alaska, hydromagmatic RÉSUMÉ. Les maars de l’Espenberg situés dans la partie septentrionale de la péninsule Seward en Alaska ont été formés par une série d’éruptions basaltiques datant du pléistocène, à travers une forte épaisseur de pergélisol. Les maars ont été creusés à une profondeur allant jusqu’à 300 m dans d’anciennes roches; d’un diamètre variant entre 4 et 8 km, ils sont les quatre plus grands maars connus sur Terre. Les éruptions hydromagmatiques qui tirent l’eau de la glace de sol sont, comme on l’a déjà constaté, extrêmement explosives. La grande capacité thermique de la glace dans le pergélisol détermine l’approvisionnement en eau qui interagit avec le magma au cours de l’éruption, donnant régulièrement lieu à un faible rapport refroidissant / combustible dans un environnement où l’eau est constamment abondante. Les maars de l’Espenberg démontrent que, dans certaines conditions, les éruptions qui déclenchent une interaction lave-pergélisol sont suffisamment puissantes pour donner naissance à des cratères de la grandeur de petites calderas. Mots clés: maar, pergélisol, arctique, Alaska, hydromagmatique Traduit pour la revue Arctic par Nésida Loyer. INTRODUCTION Although maars are the second most common volcanic landform after cinder cones (Cas and Wright, 1987), no Maars are shallow but broad craters formed by explosive previous examples have been documented of maars produced excavation into older lithologies during phreatomagmatic by eruptions through permafrost. Here we describe the eruptions. The Espenberg Maars are found on the northern- geomorphology and model the thermodynamic processes most Seward Peninsula (Fig. 1), just south of the Arctic Circle, that occur during the interaction of magma and permafrost. and farther north than any other Late Pleistocene volcanoes in Such interactions led to highly explosive eruptions and the North America (Wood and Kienle, 1990). Because of their formation of unusually large maars on the Seward Peninsula high-latitude setting in Alaska, these maar eruptions occurred of northwest Alaska. in an area where permafrost is ca. 100 m thick. The presence of basaltic rocks on the northern Seward Peninsula was first noted by Moffitt (1905). Hopkins (1959, DIMENSIONS OF HYDROVOLCANIC CRATERS 1963) identified extensive Quaternary basaltic lava flows, AND MAAR LAKES especially in the Imuruk Lake area of the central Seward Peninsula, and later described the Late Quaternary Espenberg The Espenberg Maars consist of four large, separate erup- Maars and coeval tephra layers (Hopkins,1988). Radiocar- tive craters, each excavated 100–300 m into Pleistocene bon dates and tephrochronology, together with relative age sediments and lavas. Most of the maars are circular to slightly estimates based on the degree of erosion and sedimentation in elliptical in shape, except for the Devil Mountain Maar, the lake basins, indicate that the Devil Mountain Maar is the which is an irregular composite crater containing North and youngest, formed ca. 17 500 years B.P. (Table 1); South South Devil Mountain Lakes (Fig. 1). North Devil Mountain Killeak Maar is > 40 000 years old; North Killeak Maar is Lake (5.1 km in diameter) is partly separated from South somewhat older; and Whitefish Maar may be 100–200 000 Devil Mountain Lake (3.4 km diameter) by a small sand spit. years old (Hopkins, 1988; Begét et al., 1991). Sequences of bedded surge deposits, airfall lapilli beds, 1 Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-0760, U.S.A. 2 Department of Geology and Geography, University of Massachusetts, Amherst, Massachusetts 01003, U.S.A. © The Arctic Institute of North America SEWARD PENINSULA MAARS • 63 Chukchi Sea Kitluk River Kalik DevilDevil 66°30' River MountainMountain Maar Espenberg River Singeakpuk River North North Killeak Goodhope Killeak Bay Maar Whitefish Maar South N Thick KilleakSouth Killeak tephra Maar Lava flows and cones Kongachuk 0 5 10 164°30' Creek 164°00' 66°15' km FIG. 1. Location of the Espenberg Maars on the northern Seward Peninsula, Alaska, showing Devil Mountain Maar, Whitefish Maar, North Killeak Maar, and South Killeak Maar. Surrounding each maar is a thick blanket of pyroclastic surge and airfall tephra deposits; the pattern shows the distribution of tephra more than 5 m thick surrounding each maar. TABLE 1. Radiocarbon dates on organic material buried by Devil Peninsula produced by the eruptions of the Devil Mountain Mountain Lakes Maar tephra, Seward Peninsula, Alaska. Lakes Maar all indicate an age of about 17 500 years B.P. (Table 1). The existence of only a single tephra deposit at Radiocarbon Laboratory Material Location numerous sites around the maar, the similarity of all radiocar- Date Number Dated bon dates from beneath the tephra deposits, and the absence 16 990 ± 150 B-18632 peat (macrofossil) Nuglungnugtuk River of significant stratigraphic breaks in either the distal ash or 17 420 ± 260 B-60718 peat (macrofossil) Lake Rhonda the proximal volcaniclastic sequence indicate the Devil Moun- 17 630 ± 800 W-3489 peat (macrofossil) Whitefish Thaw Pond 17 740 ± 110 B-75731 peat (macrofossil) Lumpy Drained Lake tain Lakes Maar formed during one complex eruptive episode 17 740 ± 220 B-75529 peat (macrofossil) Tempest Lake about 17 500 years B.P. The crater produced during those 17 910 ± 1501 B-18549 detrital twigs Kiliwooligoruk Creek eruptions, measured from rim to rim across the maar lake, is 16 880 ± 120 B-60716 wood (macrofossil) Egg Lake 17 980 ± 100 B-71983 sediment Egg Lake 8 km long by 6 km wide, as much as 200 m deep, and covers over 30 km2 (Fig. 1). 1 in reworked alluvium The maar craters at North and South Killeak Lakes, lo- cated 20 km east of the Devil Mountain Maar, are 4.0 and 5.0 massive pyroclastic flows, and explosion breccia can be km in diameter and ca. 20 and 12 km2 in area, respectively traced through many fresh exposures in 10–40 m high cliffs (Fig. 1). Whitefish Maar lies 15 km west of the Devil around the Devil Mountain Lakes Maar (Fig. 2a, 2b). Mountain Maar, is 4.3 km in diameter, and covers 15 km2, Stratigraphic sections were measured through the volcanic although the lake basin has been partly filled by alluvium and pile at many sites around the lakes, and complex but uninter- other sediments since the eruption. Exposures through proxi- rupted sequences of plane-bedded and cross-bedded surge mal hydromagmatic deposits are rare at the older maars, but deposits, massive explosion breccia, and scoria beds were isolated exposures exist in stream gullies at each older maar, found. No trace of nonvolcanic sediment or paleosol develop- and in each case the absence of paleosols, nonvolcanic ment was found at any level within the sequence of sediment, or other stratigraphic breaks suggests each of these volcaniclastic sediments (Fig. 2b). Similarly, radiocarbon maars formed as the result of complex but monogenetic dates from organic material preserved at numerous sites eruptive events. beneath a widespread tephra deposit on the northern Seward In comparison to the Espenberg Maars of the Seward 64 • J.E. BEGÉT et al. FIG. 2. a) Exposure of volcaniclastic sediments near the southwest shore of North Devil Mountain Lake, showing plane-bedded surges overlying massive pyroclastic flows and explosion breccia. The outcrop is 45 m high. b) Outcrop of explosion breccia near the eastern shore of South Devil Mountain Lake, containing numerous clasts of unconsolidated sands, peats, and other materials. The dumbell-shaped clast in the lower center of the photograph consists of unconsolidated, bedded medium sand, thought to have been frozen when emplaced. The knife (base of photograph) is 2 cm across. Peninsula, virtually all maars at lower latitudes are much observed elsewhere. These craters evidently mark the sites of smaller. Average diameters of typical maars are only a few explosive activity during the eruption; other explosion cra- hundred to a thousand meters, and cover less than 1 km2 in ters may have formed earlier in the eruption sequence, only to area (Cas and Wright, 1987). The largest such features be obliterated or filled in by later eruptions. Although we are previously described are about 3 km in diameter, with surface unable to find any published bathymetric data from other areas less than 8 km2 (Fig. 3). The Espenberg Maars are maars, we speculate that multiple craters may exist on other therefore several times larger in diameter than other maars, maar lake floors. Similar groups of coeval explosion craters and an order of magnitude larger in surface area. have recently been documented at sites like Cerro Xalapaxco The underwater topography of several of the Alaskan maar in central Mexico, where multiple phreatomagmatic erup- lakes was studied by depth profiling during the summer of tions occurred at a site with an especially abundant water 1992.
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